Two alternative principles that might be used in the establishment of criteria for permissible exposure to intermittent high-intensity noise are the total-immission rule and the equal-TTS principle. The former assumes that the hazard to the auditory mechanism is a function only of the integral of the acoustic power that has entered the ear (it is therefore sometimes called the total-energy principle), while the latter postulates that hazard is monotonically related to the temporary threshold shift (TTS) produced by the exposure. Because exposures of vastly different total energy may lead to the same TTS, both principles cannot be correct. Two stages of research are designed to determine the conditions under which one or the other prevails. First, normal human listeners will be exposed to various patterns of intermittent, fluctuating, and impact noises, in order to determine the equivalence of these exposures in terms of the TTSs produced. In the second stage, experimental animals (chinchillas) will be given two sets of noise exposures, one set that would produce equal, severe, but reversible TTS in humans, a second set that involves a constant total energy. Because the chinchilla is considerably more susceptible to TTS, PTS (permanent threshold shift), and observable cochlear damage than is man, these exposures will produce moderate hair-cell destruction. Evaluation of both the PTSs and the cochlear damage will indicate the relative validity of the two principles, and will permit the establishment of more reasonable damage risk criteria than the ones now generally supported on the basis of their simplicity.